Such a spool arrangement is known from U.S. Pat. No. 3,262,027 showing a solenoid structure and mounting means therefore. A magnetic yoke is associated with a tube. The tube contains an axially shiftable core. A valve member is carried at the end of said core. The magnetic yoke is used to complete a magnetic circuit through the core. It consists of two identical elements. Each element comprises a stem defining half of a curve central part of a U-shaped region. Two wings are attached to the stem and arranged into parallel spaced planes. Each wing comprises an opening. The different openings of each wing and both elements overlap. From each opening a slot extends to the edge of the respective wing. The two elements can be held together by inter-engaging of a protuberance on each wing with an opening in the adjacent wing.
It is frequently encounted that damageable parts like the spool member have to be disposed in surroundings in which they can be exposed to considerable stress. When, for example, such a spool arrangement is placed in areas where explosions can happen the spool arrangement must to some extend be able to withstand the effects of an explosion. If they can fulfill this requirement they are in compliance with European ATEX requirements for equipment being present in areas, where there is a risk for explosions. In such surroundings damageable parts have to be protected against mechanical impact. To this end, protection means are used.